Vapour compression systems, such as refrigeration systems, air condition systems or heat pumps, normally comprise a compressor, a condenser, an expansion device, and an evaporator arranged in a refrigerant circuit. Refrigerant is circulated in the refrigerant circuit and is alternatingly expanded and compressed, and heat exchange takes place in the condenser and the evaporator. Expanded refrigerant enters the evaporator in a mixed state of gaseous and liquid refrigerant. As the refrigerant passes through the evaporator, it evaporates while exchanging heat with a secondary fluid flow, such as an air flow, across the evaporator. In order to utilise the potential refrigerating capacity of the evaporator to a maximum extend, it is desirable that liquid refrigerant is present along the entire length of the evaporator. On the other hand, it is undesirable that liquid refrigerant passes through the evaporator and into the suction line, since it may cause damage to the compressor if liquid refrigerant reaches the compressor. It is therefore desirable to control the supply of refrigerant to the evaporator in such a manner that the compressor is not damaged while at the same time utilising the full capacity of the evaporator.
In order to obtain this, a pressure sensor is typically used to measure a pressure to derive an evaporation temperature of the refrigerant and a temperature sensor to measure the outlet temperature of the refrigerant leaving the evaporator. Traditionally, the expansion valve is controlled based on a temperature difference between this evaporation temperature and the outlet temperature at the pressure in question.
Thus, it is necessary to use a pressure sensor in the above-described control. As such a pressure sensor may fall out or malfunction, it may be impossible to derive at an evaporation temperature, which may further prevent control of the supply of refrigerant to the evaporator until the pressure sensor is restored.
US 2004/0068999 discloses a controller for an expansion valve of a refrigeration system. Sensors may register various selected temperatures and pressures of the cooled medium and refrigerant at different positions in the refrigeration system. The measured pressures and temperatures are used in the controller for controlling the injection of refrigerant into the evaporator in order to maintain stable operation conditions.